Over-voltage detecting and protection apparatus



United States Patent Inventor Robert Adler N orthfield, Illinois Appl. No. 712,411 Filed March 12,1968 Patented Dec. 1, 1970 Assignee Zenith Radio Corporation Chicago, Illinois a corporation of Delaware OVER-VOLTAGE DETECTING AND PROTECTION APPARATUS 8 Claims, 6 Drawing Figs.

US. Cl 200/181, 313/146, 3 17/31: 328/9 Int. Cl .....l-l01h 59/00 Field ofSearch 313/146,

56] References Cited UNITED STATES PATENTS 2,786,111 3/1957 Reed,.lr. 200/181 2,927,255 3/1960 Diesel 200/181 Primary Examiner Raymond F. Hossfeld Arlorneys- Francis W. Crotty and David P. Ogden ABSTRACT: An electrostatic overvoltage protection vacuum tube apparatus and voltage indicator is useful in a fail-safe overvoltage control system for image-reproducing cathoderay tubes operating at high voltages of about 25kv. A tractably mounted electrode having an extended surface facing a similar electrode surface responds to an electrostatic field caused by a voltage greater than about 25kv. to provide an indication of the voltage magnitude and/or to reduce the high voltage or interrupt its application to the cathode-ray tube.

fiATENTEU'nEm I970 SHEET 1 OF 2 INVEN'IOR. o -v n igl al Robert Adler PATENIEDuEm I970 SHEET 2 OF x Displacement Voltage Robert Ad I er BY Attorney OVER-VOLTAGE DETECTING AND PROTECTION APPARATUS BACKGROUND OF THE INVENTION This invention pertains to an overvoltage protection apparatus, and more specifically, to such apparatus comprising an overvoltage switch operable by direct application of an electrostatic potential difference between electrode surfaces.

Cathode-ray tubes of the type used in television receivers have electron accelerating voltages of about 25 kv. When electrons are accelerated by high voltages and impinge upon certain materials, X-rays are produced. Therefore, it is considered of some hazard to have voltages greatly in excess of 28 kv. in certain portions of television receivers.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide an improved overvoltage protection apparatus.

Another object of the present invention is to provide an improved overvoltage protection device directly responsive to electrostatic fields.

Yet another object of the present invention is to provide an improved high-voltage indicating and protection device.

In a television receiver having an image display tube, a potential of the order of 25 kv. is supplied thereto by a highvoltage source. In accordance with one embodiment of this invention, an overvoltage detecting and protection apparatus includes an evacuated envelope having a pair of terminals, coupled to the high-voltage source. A first electrode in the envelope has an extended surface connected to one terminal. A second electrode tractably supported in the envelope has an extended surface facing the first electrode surface and connectedto the other terminal, to be movably attracted in accordance with the magnitude of an electrostatic field between the surfaces. Means are coupled to the electrodes the highvoltage power supply in response to a predetermined movement of the second electrode at voltages substantially greater than 25 kv.

In each case, the means for modifying the operation of the high-voltage power supply comprises a normally ineffective control means, such as a fuse or a supplemental load, actuatable to regulate or interrupt the power supply, and means such as a pair of switch contacts responsive to a predetermined movement of the second electrode at voltages substantially greater than about 25 kv. for actuating the control means.

The features of the present invention which are believed to be novel are set forth with particularly in the appended claims. The organization and manner of operation of the invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings, in the several figures of which like reference numerals identify like elements, and in which:

FIG. 1 is a schematic illustration of one embodiment of the present invention;

FIG. 2 illustrates another embodiment of the present invention;

FIG. 3 is a simplified perspective view of another embodiment of the present invention;

FIG. 4 graphically illustrates the response of an electrode of the present invention to the applied electrostatic field;

FIG. 5 is a curve illustrating the relationship between the resonant vibrational frequency of a tractable electrode and the electrostatic field; and

FIG. 6 is a simplified illustration of yet another embodiment of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1 an envelope of a tube 10 surrounds a region which is electrostatically insulative. Certain pressurized gases and certain liquids provide adequate electrostatic insulation. However, a simple evacuated envelope is preferred. A lower portion of the vacuum tube 10 is a solid base platform 12 supporting elements having critical spatial'relationships. It should be appreciated that thesolid base member 12 can have a rigid structural member extending upward to provide similar critical location of electrodes in the upper portion of the tube 10. A high-voltage power source 11 provides a 8+ voltage of about 25 kv. to a high-voltage terminal cap 13 connected to a high-voltage electrode 14 within the tube 10. The electrode 14 has parallel extended flat surfaces on the inner sides of spaced members 15. Centered between these surfaces is a second extended electrode 16 tractably supported on a flexible reed 17 and connected to a ground terminal 18. When the power source 11 provides 25,000 volts, a corresponding electrostatic field is developed between the parallel extended surfaces of the electrodes 14 and 16. Usual care should be exercised to eliminate sharp edges subject to field emission.

Such an electrostatic field E produces an attraction force P which is proportional to the square of the voltage V applied between the facing surfaces. As no current is flowing in the tube 10, 8+ voltage equals V. P isalso inversely proportional to the square of-the distanceD between the parallel surfaces as follows:

wherein s is the dielectric constant of vacuum and A isthe electrode area. So long as the flat electrode 16 remains centered between the fiat members 15', this equation (1) may be written as There are two equal and opposite electrostatic forces acting on the reed, both of magnitude P, and they cancel out when electrode 16 is centered. If, however, electrode 16 is moved to one side, say to the right, the force of attraction on the right P, increases and the force of attraction on the left P decreased, producing a net force P, =P P,. For small displacements, P, is proportional to the displacement dx and is given by The quantity 4P/D is analogous to a similar quantity encountered in balanced magnetic structures used' in microphones and earphones, where it is called the negative stiffness S Reed 17 is selected to be flexible andhas a predetermined mechanical stiffness S In the presence of a voltage applied to electrode 14, the negative stiffness S is algebraically added to the mechanical stiffness 8,, resulting in a residual stiffness s which is a function of the applied voltage V:

o nez( According to the invention, the mechanical stiffness S is so chosen that at a predetermined voltage V,,, the residual stiffness becomes zero:

S'.,=2e,,AV,, /D whence D) o ne2( D) With the electrode 16 centered, the flexural stiffness of the reed 17 may thus be reduced to zero by an electrostatic field produced by a potential such as 30 kv. When this happens, the

electrode 16 snaps to one side to short out the high voltage. Removal of the high-voltage electrostatic field allows the reed to regain its stiffness and return the electrode 16 toward the center. Assuming the high voltage is reestablished quickly, the electrode 16 will oscillate rapidly between the spaced members 15.

The electrode 16 is flexibly supported on the reed 17. Alternatively, a similar reed 17 may be extended through the gap of the electrode 14 bifurcation and be torsionally strained by tilting of the electrode 16. The electrostatic field responses of a torsional support system are similar to the flexible system shown.

Also shown in FIG. 1 are a pair of fixed contacts positioned to engage the reed 17 prior to the electrode 16 coupling the B+ voltage to the ground terminal 18. The contacts 20 are connected in circuit with a voltage dropping resistor 21 which cooperates with a power supply impedance (indicated as a resistor 21') to reduce the high voltage when the switch 17-20 is closed. The relation between the magnitude of resistor 21 and the position of the contacts 20 is selected so that when an overvoltage causes closing of the switch the electrodes are close enough so that P will maintain the switch 17-20 closed despite the voltage drop across resistor 21. Thus, the switch 17-20 will maintain a reduced voltage until the circuit is manipulated to return to normal operation. The high-voltage output of the source 11 is adjusted manually by manipulation ofa voltage control knob 19. Also, the overvoltage protection apparatus of FIG. 1 will effect voltage reduction until the television set is turned off. In accordance with this invention, a serious overvoltage condition is prevented by positive action ofa device directly responsive to the high voltage.

FIG. 2 shows a multiple electrode overvoltage protection device wherein the centering of the low-voltage electrodes 16' within the structure of the electrode 14 is not as critical as that shown in FIG. 1. This is because the tractable electrodes are each attracted to one of the proximate surfaces. Their being offcenter will cause a slightly greater attraction of one and a similar reduced attraction of the other.

The low voltage electrodes 16 are coupled by a pair of contacts 22 which form a normally closed switch 22-22. To attain such condition the reed supports 17 are initially tensioned toward the switch-closed position. As the low voltage electrodes 16' respond to the increased electrostatic field, they are pulled toward the members 15 and thereby away from one another to open switch 22-22. When closed, the switch couples terminals 18 and 18' to shunt a high impedance resistor 23 to reduce the voltage drop thereacross and eliminate an overvoltage signal".

FIG. 3 is a perspective view of another multiple electrode three-terminal apparatus wherein the low-voltage electrodes 16a are curved to surround a single-plate high-voltage electrode 14a. Although electrode 14a is illustrated as a flat plate, its being bent to a central cylindrical form results in more precise application of the following theory. 'A substantially cylindrical inner surface of the low-voltage electrodes 16a provides improved centering tolerances. Slight misalinement will cause one of the low voltage electrodes to be attracted somewhat more and the other somewhat less, with relatively little net change in the voltage applied to electrode 14a to cause their normally open switch contacts 22 to close.

In the device shown in FIG. 3, a large electrostatic attraction force P of the type developed by about 28 kv., will cause the switch 2222 to shunt a load 24 supplied through a circuit breaker or slow-blow fuse F. After a period such as 30 milliseconds, the fuse F ruptures, and the load 24 is per manently disconnected from a power source providing a C+ voltage. By way of example, the load 24 includes a screen grid or a heater 25 ofa vacuum tube 27 indicated in FIG. 1 and essential to the operation of the high-voltage source 11. Blowing of the fuse F permanently disables the high-voltage source. A manual operation of replacing the fuse F is required to restore normal B+ voltage.

The power supply furnishing energy to the load 24 normally provides a relatively low C+ voltage such as .5 to-50 volts which is insignificant compared to the 2530 kv. electrostatic field developed between the electrodes 14a and 16a. A similar voltage signal level is utilized in the device illustrated in FIG. 2, which signal may also be used to operate a relay or circuit breakenetc.

In order to calibrate the device of FIG. 3 to provide the voltage limit signal at precisely a desired voltage limit such as 27.5 kv., a trimming magnet 30, indicated as a permanent magnet, is secured to the envelope 10 adjacent to one of the low-voltage electrodes which includes a magnetic material. The magnetic flux regulates the rest position of the reed 17 supporting one of the electrodes 16a. Moving magnet 30 up and down along the tube surface provides adjustment of the rest'position.

FIG. 4 is a graph'wherein the scale of the abscissa is voltage in kilovolts and the scale of the ordinate shows displacement of the movable electrode in millimeters. Curve A illustrates the displacement of the electrodes 16a of FIG. 3 as the electrostatic force builds up between the electrode surfaces. With a surface to surface spacing of approximately 6 mm., the displacement of the tractable electrode remains small untilthe voltage magnitude builds up to a value such as 20 to 25 kv. At 27.5 kv. the voltage limit switches 22' close. The displacement of the electrodes 16' of FIG. 2 follows a curve B, with the prestress of the supporting reeds supplying a force which is overcome only when the voltage reaches 27.5 kv.

Because of the fact that the electrostatic force P is inversely proportional to the square of the distance D between the electrically charged surfaces, electrode motion tends to become unstable after a substantial displacement has occurred. At the point of instability the movable electrode begins to move rapidly within the electrostatic field until it strikes a supporting structure. Of course, the electrode structural arrangement shown in FIG. 3 does not become unstable because of structural support the electrodes provide each other at voltages of 27.5 kv.

Because the electrostatic field E decreases the residual stiffness 5,. of the reed, it also reduces the resonant frequency m at which a reed electrode tends to vibrate. At different electrostatic field magnitudes, the stiffness S of the system varies as follows:

Neglecting the mass of the reed 17, the unmodified resonant frequency o of the reed supported electrode 16 is given by:

where T is the thickness and p is the density of the material. As the stiffness is reduced (0 decreases proportionally. Therefore (w/w.) =1V /v. (1

which is the equation of a circle as shown in FIG. 5, wherein the scale of the abscissa is the electrostatic field-producing voltage in kilovolts, and the scale of the ordinate is the resonant frequency in hertz.

Assuming the structure shown in FIG. 1 with a two-position switch 31 normally shunting an insulation insert 32 in the bifurcated electrode 14, when the switch 31 is moved to the second position, a cycle power source 33 is applied across a transformer 34 to couple the two members 15. When the two members 15 have approximately 100 volts alternating potential impressed thereacross, along with the 25 kv.

. unidirectional potential, a 60 hz. vibrating force is applied to the central electrode 16.

In a reed system as discussed above, the negative stiffness of the system cancels the stiffness of the reed at approximately 30 kv. to attain a voltage limit signal as shown in FIG. 5. However, it is advantageous to be able to define accurately the desired voltage magnitude of 25 kv. so that the voltage may be adjusted in the field without use of expensive voltage checking equipment. Thus the electrode and support system 16-17 is selected to have a resonant frequency of 60 Hz. at 25 kv. Onev set of parameters that will provide such resonance is a 5-mil thick sheet steel (p=8 X10) member 16-17 about 2 inches long. As illustrated in FIG. 5, such a construction would have a natural resonant frequency (without any electrostatic field modification) of approximately I09 l-Iz.

A similar voltage-detecting device is shown in FIG. 3 with relative vibration between electrodes 16a and the high-voltage electrode 14a in response to the 60 Hz. modulated 25 kv. applied by means of a two-position switch 31. Compared to the 25 kv. the 110- volt fluctuating signal is small and will not cause a large deflection of the Vibrating electrode (16 or 14a). However, the small deflection will be quite noticeable and will sharply define the unidirectional potential magnitude of 25 kv. Of course, the overvoltage protection device used in the overvoltage apparatus can be of forms different than that shown in FIGS. 1,2 and 3.

Referring now to H0. 6, both electrodes 14b and 16b are tractably mounted on similar reeds 17 with the base 12 having an insulating projection 35 which will prevent these electrodes from making contact and provides a large surface distance between the electrodes coupled-across the 25 kv. A contact 22 secured to the low-voltage electrode 16b cooperates with a contact 36 supported on a second reed 37 to form a normally open switch 22-36. Other switch contact 38 supported on the reed 37 and contact 20 which is fixed provide another normally open switch 20-38. Attraction between the electrodes 14b and 16b first causes a change of switching condition in the switch 22-36 and later causes a change of switching condition of the switch 20-38. As illustrated the switch 22-36 is in circuit through pins 50 and 51 with an indicator lamp 40 which provides a signal change at the desired 25 kv. voltage level. The voltage limit signal occurs at 27.5 kv. in accordance with the closing of switch 20-38 through pins 52 and 51. Thus, when switch 22-36 is closed, a circuit is completed from C+ through indicator lamp 40, contact 22 and contact 36 to ground, thus lighting indicator lamp 40. When switch 20-38 closes at a higher supply voltage, the circuit from C+ through resistor 41, contact 20 and contact 38 to ground is completed, developing a voltage across resistor 41 which is an overvoltage signal which can be used in any desired way. Obviously the several contacts could be located so that the switches were normally closed instead of normally open with corresponding circuit changes. Also, a voltage-dropping resistor 41 can be a fuse or circuit breaker ofthe type illustrated in FIG. 3.

The movement of the tractably mounted electrode 16b is illustrated in curve C of FIG. 4. The initial movement of the electrode 16b is similar to that shown as curve A except that the curve C rises somewhat more rapidly because electrode 14b moves toward it to further reduce D At point M an abrupt change of slope occurs where the switch 22-36 is closed. Thereafter, the electrostatic attractive force is required to bend both reeds l7 and 37. At 27.5 kv. the switch 20-38 is closed to provide the overvoltage signal.

In the construction of FIG. 6, the initial spacing of the electrodes is regulatable by securing the trimmer magnet 30 near the electrode 14b which would include magnetic material. At any time a trimmer magnet is used, it should be secured in such a way that it cannot be easily moved. In this way the go, no-go regulation of the present invention provides a fail-safe voltage limit.

FIG. 6 also shows the several elements supporting the contacts 20, 22, 36, 38 as being precisely spaced prior to being secured to the rigid base plate 12. The precise spacing is assured by compressed insulating sheets 42 firmly held by a rivet 43 and an insulating washer 44. in this manner the critical gaps of the switches 20-38 are maintained within very close tolerances to provide signals at 25 kv. and 27.5 kv.

While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.

lclaim:

1. in a television receiver of the type including an image reproducing cathode-ray tube having an element requiring a high unidirectional potential relative to another element and a high voltage power supply for providing a voltage of the order of 25 kv. across these elements, an overvoltage detecting and protection apparatus comprising:

an evacuated envelope having terminals extending therefrom, with a pair of terminals receptive of the high voltage;

a first electrode insaid envelope having an extended surface and connected to one ofthe pair of terminals;

a second electrode tractably supported in said envelope to have an extended surface facing the first electrode surface and connected to the other ofthe pair of terminals to be movably attracted in accordance with the magnitude of an electrostatic field between the surfaces;

normally ineffective control means actuatable to modify the operation of said power supply; and

means responsive to a predetermined movement of said second electrode at voltages substantially greater than about 25 kv. for actuating said control means.

2. An overvoltage detecting and protection apparatus as in claim 1, further comprising:

other means for effecting low frequency electric field generation to cause at least one of said electrodes to vibrate at such low frequency when a particular magnitude of unidirectional voltage is applied across said first and second electrodes, such vibration providing a visible voltage magnitude indication within said envelope.

3. An overvoltage detecting and protection apparatus as in claim 1, further comprising:

movable magnet means associated with at least one of said electrodes for determining the voltage at which said actuating means becomes effective.

4. An overvoltage protection vacuum tube device, comprising:

a high-voltage electrode having extended surfaces connected to a high-voltage terminal;

a pair of low-voltage electrodes tractably supported to have an extended surface of each facing one of said first electrode surfaces and each connected to a low-voltage terminal; and

switch contacts mounted on each of said low-voltage electrodes to change switching condition at a predetermined maximum voltage applied across said highand low-voltage electrodes.

5. An overvoltage protection device as in claim 4, wherein said low-voltage electrodes are positioned on opposite sides of said high-voltage electrode so that the field developed therebetween pulls the low-voltage electrodes together to change the switching condition.

6. An overvoltage protection device as in claim 4, further comprising:

a magnet developing a flux pattern including at least one of said electrodes which one is magnetic, for determining the potential at which said contacts change switching condition.

7. An overvoltage protection vacuum tube device, comprising:

a rigid support member;

a high-voltage electrode having an extended surface supported on said support member and connected to a highvoltage vacuum tube terminal;

a low-voltage electrode tractably supported on said support member and forming an extended surface facing said high-voltage electrode surface and connected to a lowvoltage vacuum tube terminal;

a first switch having a contact on said low-voltage electrode;

another contact of said first switch facing said first contact on a movable support mounted on said support member and electrically coupled to another vacuum tube terminal; and

a second switch having a fixed contact coupled to still another vacuum tube terminal and a movable contact on said second contact support whereby the developing of a predetermined electrostatic field between the electrode surfaces causes said low-voltage electrode to move to change the switching condition of one of said switches and a greater electrostatic field causes further movement to change the switching condition of the other of said switches. I 8. An overvoltage protection device as in claim 7, wherein 

